In gas and steam turbines it is known to provide the rotor blades with plug-in fingers and to connect them to the head of the disk wheel via several rows of bolt and hole connections. In modern steam turbines, after a certain operation time, it happens that excessive radial friction forces prevent bolts or rivets (of up to 400 mm length in final stages and shorter bolts in high temperature stages) from being removed from the holes merely by applying pressure. This is due to the internal stresses between different levels inside the bolt, to contamination and oxidation of the mating surfaces and to elastic transverse expansion of the bolts when subjected to compression in combination with a jamming and latching effect due to disk or blade stems which are bent and slanting.
Since the known methods for removing jamming shrink fittings, such as subcooling or heating, and such as applying pressurized hydraulic oil with simultaneous pressing and drawing, assisted by a pneumatic hammer, are either technically inapplicable or else, if they can be applied, do not allow the desired result to be obtained, bolts have in the past been drilled out partially or totally down to a remaining wall thickness of about one tenth of a mm in order to release the internal stresses. However, there is a great risk that the rotary drill deviates and after penetration into the softer material of the hub is pushed progressively aside by the harder material of the bolt. As a result, holes have been observed in the stems of rotary blades and wheel disks having an ovality of 2 to 3 mm, which effect is detrimental for the connection function. In spite of the fact that the bolt drilling out might be rendered safer by the use of drilling sleeves and by operating with high precaution, this method cannot be applied in practice due to its relatively large time consumption and the resulting long turbine down-time.
It is therefore an object of the invention to provide a method of the kind indicated above and also devices for implementing this method allowing an easy and rapid removal of cores, especially shrink-fitted plug-in fingers.